Fluororesins are excellent in such characteristics as heat resistance, oil resistance, chemical resistance, mold release characteristics, lubricating properties and liquid chemical impermeability. However, fluororesins are expensive and are inferior in moldability, workability, mechanical characteristics and creep characteristics, among others. To solve these problems, resin laminates comprising a fluororesin layer and a non-fluororesin material layer have been proposed.
For example, Japanese Kokai Publication H05-8353 discloses a laminated tube comprising an outer layer made of a polyamide resin and an inside layer made of a fluororesin. This patent document discloses a technology comprising introducing a crosslinking structure between the polymers constituting the polyamide resin layer and fluororesin layer by irradiation with radiation to secure the interlaminar bond strength between both the layers.
However, this technology, which consists in irradiating the surface of the moldings obtained by molding of a fluororesin with radiation and then bonding thereto a polyamide resin layer in the manner of lamination, is complicated in procedure and inferior from the economical viewpoint since it requires an irradiation apparatus.
To solve these problems, a method has been proposed which comprises bonding a fluororesin layer to a non-fluororesin resin material layer in the manner of lamination by coextrusion of both materials to produce a resin laminate such as a multilayer tube.
For example, Japanese Kokai Publication H07-53823 discloses a method of producing three-layer type multilayer tubes having a polyamide resin layer as the outside layer and a fluororesin layer as the inside layer using a multilayer tube coextrusion machine which method comprises using a layer of a resin composition comprising a specific polyamide resin and a specific fluororesin as an intermediate adhesive layer.
However, that method has a problem. Namely, due to the use of a polyamide resin and a fluorine resin, which are poor in mutual compatibility, as the adhesive layer, the morphology of the adhesive layer varies depending on the molding conditions, use temperature and other environmental conditions, so that the cohesive force within the adhesive layer and the bond strength between the adhesive layer and the neighboring layers change accordingly, hence no quality stability can be secured.
To solve this problem, an idea of modifying the fluororesins themselves was conceived, and various cohesive fluororesins have been proposed. For example, the international patent application WO 99/45044 discloses a fluorine-containing ethylenic polymer having a specific carbonate and/or haloformyl group content as a fluororesin capable of being bonded to polyamide resins. However, there is a problem in that even the use of this fluorine-containing ethylenic polymer results in unsatisfactory levels of bond strength.
Meanwhile, in the field of semiconductor manufacture, fluororesins are used as tubes, wafer carriers, and lining materials for containers for high purity liquid chemicals or for storage tanks, among others. In recent years, liquid chemicals supplemented with a surfactant, ozone-containing water and like liquids low in surface energy have been put to use in the field of semiconductor manufacture for the purpose of increasing the detergency. These liquid chemicals are highly penetrative into fluororesins and cause cracking thereof.
Further, when they come into contact with such liquid chemicals as solvents, inorganic acids, alkaline aqueous solutions, gasoline, and ozone-containing water with the stress hung, fluororesins are susceptible to cracking, and this is a matter of concern especially when they are used in the form of piping tubes in automobiles and the like or tubes for transport liquid chemicals, among others.
A method of improving the resistance of fluororesins to such stress cracking (stress cracking resistance), which is in current use, comprises increasing the molecular weight of the tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer [PFA], for instance, used in the form of such injection moldings as wafer carriers, joints and nuts in the field of semiconductor manufacture to thereby improve the strength thereof.
However, the stress cracking resistance of fluororesins will not be satisfactorily improved in spite of increases in molecular weight but rather the viscosity thereof becomes very high and the critical shear rate decreases, so that the productivity lowers. Another problem is that moldings obtained by injection molding, among others, show a defective appearance called melt fracture.
Thus, in the art, there are no fluororesins available capable of providing moldings or laminates excellent in moldability, productivity, interlaminar bond strength and stress cracking resistance without impairing such characteristics as chemical resistance, oil resistance, low liquid chemical permeability and heat resistance.